Dual range frequency-modulated radio altimeter



April 25, 1950 c. J. H. A. STAAL ET AL 2,505,692

DUAL RANGE FREQUENCY-MODULATED RADIO ALTIMETER 2 Sheets-Shet 1 Filed May9, 1946 D (URNZZJSJOMWS/[ZWHKJJAWOMMSMAZ ILEWZIR/Uf Vii/U52 WM.

' INVENTORS.

ATTORNEY Patented Apr. 25, 1950 UNITE DUAL RANGE FREQUENCY-MODULATEDRADIO ALTIMETER,

Cornelis Johannes HenricusAntonius Staal and Hendrik van de Weg,Eindhoven; Netherlands, assignors, by mesne assignments, to'HartfordNational Bank and Trust Company, Hartford,

Conn, as trustee Appiication May 9, 1946, Serial No. 668,532

In the Netherlands June 10, 1942 Section 1, Public Law 690, August8,1946

Patente'xpires June 10, 1962 Claims. (Cl. 343 14) takes place bybringing the maximum frequency deviation of the frequency modulatedoscillations to be transmitted to a higher and lower value respectively.

The transition to another measuring range preferably takes place inaccordance with the average beat frequency between the emittedoscillations and the oscillations .receivedafter reflection.

With distance meters of the kind to which the present invention relatesit has already been proposed to effect the transition from oneof themeasuring ranges of the indicating device, indicating the distance to bemeasured; to a neighbourin'g'range automatically in accordance with avoltage depending on the distance to be measured. In thiscasethechange-over of the measuring range is effected by modification ofthe sensitiveness of. the indicating instrument to which the beatfrequency is supplied.

The expedient according to the invention for changing over the measuringrange has the advantage over the devices referred to that for at leastapart of the distances to be measured the bandwidth required for"transmitting the beat between the emitted. oscillations and thereflected. oscillations is smaller so that with the same amplificationthe noise is weaker.

In fact, withia distance meter of the type according tofthe. presentinvention the average frequency of the beat between theemitted oscilla-itions and the oscillationsreceived after'refiection' increasesproportionally to the distance which requiresthatthe device fortransmitting thevbeat musthav'e a bandwidth corresponding,

curring'in. the. amplifier, which is considerable.

" denser .8 forms part of .a tuned circuit.of thetransmitter asv aresult-ofgwhich the frequency... of the oscillationsproducedbythetransmitter, varies in accordance with the frequency'of'thel 2, owing to the large bandwidth which the amplifier. must have.

Inasmuch as the extentof frequency deviation, in a radio altimeter inaccordance withthefinr" vention, is reduced when switching from one,

altitude measuring range to the next higher.

range, the amplifier bandwidth required'for passing the beat resultingin the higher rangeis narrower. Consequently, with thesamedegreej ofamplification. a more favorable,signabtoenoise ratio is obtained with.the narrowerfbandwidtht in the higher rangeasagainstthe broaderba ndwidth in the lower range. Theinvention will be more fullysexplained byreference to the accompanying drawing, given by. way of. example, inwhich Eigure 1 represents. one form of construction of a distancemeterfaccording to the invention, in which the changeover of the measuringrange takes placemanually,

Figures 2,3 and 4 representing curvesby means ofwhich the operationofthe device shown inv Figure 1 willbe explained and Figure 5 showing, oneform of construction of a device according to, the invention withautomatic change-oven of the measuring range.

In theform of construction shown in Eigure 1. frequency modulatedoscillations are emitted at, the measuring station by a transmitter Ithrough an antenna 2 in thedirection of=a re-, fleeting surface 3 orwhich. the distancehhasto, be determined relative to. themeasuringstation.

The frequency modulation or the transmitter is obtained by means of afrequency modulator 4 consisting of a magnet system 5 whichisenergisedby an alternating current, for instanceoi 50 cycles,originating from a supply 6. This ma net. system causes a periodicaloscillating. mo

tion of a movable condenser electrode] of a con:

denser 8 by which isbrought. abouta capacity,

variation of this condenser. The said v conI-I supply 6t The emittedfrequency modulated, oscillations.

strike thesurface .3 where they are, reflected.

The oscillations, received after. reflection by. a receiving antenna 9,together with oscillations appearing in the antenna 9 bydirect-radiation ofjthe. antenna 2; aresuppliedto a mixing stage Iii.Owing. tothe difference in length of the pathsv coveredby theoscillations received by; direct radiation, and theoscillationsreceivedafter e; flect om a equen y fie en e tea il -ex sts between thetwo oscillations, the average of which difference frequency is a measureof the distance between the measuring station and the reflectingsurface. The beat frequency appearing in the output circuit of themixing stage is supplied, through an amplifier II, to a frequency meterl2 which is provided with an indicator l3 giving an indication whichis'proportional to the distance to be determined.

The device referred to has two measuring ranges of which the lower rangeextends from to 100 meters and the higher range from 100 to 1000 metersand more.

According to the invention change-over to the higher and lower rangerespectively of the two measuring ranges is effected by bringing themaximum frequency deviation of the frequencymodulated oscillations to betransmitted to a lower and higher value respectively. To this end thecircuit, through which the oscillation produced by the supply 6 issupplied to the magnet system comprises a, resistance l 4 which isshortcircuited by a switch l5 in determining distances lying within thelower measuring range and whose short-circuit in determining distanceslying within the higher measuring range is removed by opening the switch15. The removal of the short-circuit of the resistance l4 causes themagnet system 5 to be energised with a smaller current due to which theamplitude, with which the condenser electrode 1 moves to and fro,becomes smaller. A smaller amplitude of the movement of the condenserelectrode 1 bring about a smaller capacity variation of the condenser 8as a result of which the maximum frequency deviation of thefrequency-modulated oscillations emitted by the transmitter I decreases,as is shown in Figure 2 where the curve indicates the maximum frequencydeviation as a function of the distance to be determined for a distancemeter having two measuring ranges. As long as the distance to bemeasured is within the lower measuring range and lies between 0 and 100meters the maximum frequency deviation is f1 whereas in the case thedistance to be determined exceeds 100 meters the maximum frequencydeviation amounts to is.

In Figure 3 the average beat frequency fz, which occurs in the outputcircuit of the mixing stage [0, is indicated as a function of thedistance D to be determined. When this distance lies between 0 and 100meters the average beat frequency increases with an increasing distance;when the distance to be determined exceeds 100 meters the short circuitof the resistance I4 is removed by means of the switch l5 as a result ofwhich the beat frequency, owing to the smaller maximum frequencydeviation of the transmitted frequency-modulated oscillations, isreduced to a smaller value in, which value increases continuously withan increasing distance between 100 meters and say 1000 meters.

To make this more clear Figure 4 represents the frequency f of thetransmitted frequencymodulated oscillations as a function of the time t.If the frequency of the unmodulated carrier wave is f0 then thefrequency will vary as a function of time, due to the modulation by thesupply 6 with the resistance I 4 being short circuited, as is shown bythe curve a. Oscillations emitted by the transmitter I at the moment i=0are received after a certain time t=1 which depends on the distance tobe determined. In this time 1 the frequency of the frequency-modulatedoscillations produced by the transmitter is altered and. amounts, forinstance, to fol as a result of which a maximum beat frequency fc1-fcappears in the receiver.

With a distance of say meters the maximum beat frequency would then be,for instance, ,fc2fc- If no change-over were to take place the maximumbeat frequency would steadily increase with an increasing distance and,for instance with a distance of 1000 meters, amount to fc3-fc, in whichcase a bandwidth of fcB-fc is necessary in the receiver.

If, however, with a distance of 100 meters the short circuit of theresistance i4 is removed the frequency of the transmitted oscillationswill vary as a function of time due to modulation by the supply 5, as isshown by the curve I) in Figure 4,

which curve shows a smaller maximum frequency deviation than the curvea.

Under these conditions the maximum beat frequency with a distance of 100meters will be fci-fc, which maximum beat frequency increases with anincreasing distance and at a distance of say 1000 meters amounts tofc5fc. In this case only a bandwidth of fc5fc is required in thereceiver, which is much smaller than if no changeover of the maximumfrequency deviation would take place.

In the device according to the invention the measuring range may bechanged over either by hand, as is shown in Figure 1, or automaticallyin accordance with the average of the beat frequency between the emittedoscillations and the oscillations received after reflection.

In the device shown in Figure 1, in whichchange-over is effected byhand, the short circuit of the resistance I4 is removed or reduced bymeans of a relay [6 which is included in a circuit comprising a supplyI! and a manually operated switch 18, the relay in determining distanceswithin the higher measuring range being energised by closing the switchl8 and the short matically in accordance with a voltage depend-- ing onthe average beat frequency this control voltage is preferably taken fromthe output circuit of the frequency meter I2, in which appears a voltageproportional to the average beat frequency, which voltage as a functionof the distance to be measured has a similar variation as is indicatedby the curve in Figure 3 and, with an increase exceeding the valuecorresponding to the largest distance within the lower measuring range,can be used for removing the shortcircuit of the resistance l4, whereaswith a decrease below the value corresponding to the smallest distancewithin the higher measuring range'can be used for bringing about theshort circuitof the resistance [4 by means of the relay l6.

If the control voltage in question is directly included in theenergising circuit of the relay IS the following phenomenon may occur onpassing to another measuring range.

When, for instance, with an increasing distance between the measuringlocationand the reflecting surface the largest distance within the lowermeasuring range is attained the relay is,

accesses since the control voltage takenfrom the=fre-:- quency'meter l2exceeds the response voltage of-- the relay, isenergised and the shortcircuit ofthe l resistance M is removed as a result of whichthemaximumfrequency deviationof the oscillations to be transmitted is reduced. 7This involves a decrease of the averagefrequency of the beat oscillationand consequently also a decrease of the control voltage which appears inthe output circuit of the frequency meter [2 and is supplied to therelay IS. The decrease in control voltage caused by changing-overthemeasuring range" may'ha-ye sucha value that this voltage falls belowthe de-energisation voltage of the relay,-

as a result of which the relay i8 is de-energised immediately afterchanging-over the measuring range and removes again the" short circuitof the resistance l4, by which the change-over just effected is offsetagain. Sincahowever, the distance to be measured, is larger than thelargest distance in the lower measuring range the relay [6 is energisedagain and the process set out above is repeated periodically.

In order to avoid this undesirable phenomenon the changing-over of themeasuring range in the circuit arrangementshown in Figure 5 is effectedby means of a memberwhich, is controlled by the output voltage of thefrequency meter l2 andwhose sensitiveness, before changing-over themeasuring range, is changed in such manner that the change-over is notoffset by the variation of the control voltage which, appears onchangingover the measuring range.

In this form of construction the said member is constituted by a relayl9 which is inserted inrthe output circuit of a discharge tube ZfitoWhosegrid the. output voltage of the frequency meter I2 is supplied witha negative polarity. .For varying the sensitiveness of the relay IS aresistance 2| is provided which by means of a breakcontact 22 can beconnected in parallel with the coil of the relay. A second breakcontact23 ofthe relay [9 is included in an electric circuit comprising a supplyI! and the relay !5 which upon energisation removes the short circuit ofthe resistance It In determining distances within the lower measuringrange the negative control voltagev suppliedto the tube 28 has such asmall value that-the anode current of the tube is suflicient to operatethe relay I9. However, with an in, crease of the control voltage toabove the value corresponding to the largest distance within the lowermeasuring range this is no longer the case and the relay I9 is released,as a result of which the contact 22 connects the restistance in parallelwith the relay Hi, the contact 23 closing the circuit of the relay i6.By energisation of the relay IS the short circuit of the resistance I4is removed which involves a decrease of the received beat frequency; thenegative grid voltage supplied to the grid of the tube 29 decreases andthe anode current of the tube increases. However, by connecting theresistance 2! in parallel with the coil of the relay [9 thesensitiveness of this relay is reduced to such a degree that the relayis not operated in spite of the increase in energisation current, sothat the change-over of the measuring range by removing the shortcircuit of the resistance I i cannot be undone by the variation of thecontrol voltage of the relay 19 which occurs in changing-over. In thiscase it is essential that the decrease in sensitiveness of the relay i9occurs before changing-over of the measuring range takes place byreducing the maximum frequency deviation of the transmittedoscillations; in connection. therewithi: thecrelay l6 performing thechange-over is preferably.

retarded:

In determining distances within the higher.

measuring range-the relay i9 is not operated despite the factthat'anode, current flows in the anode circuit -of thetube 29; If,however, the distance to be measured falls below the smallestdistancewithin the higher measuring range,

which distance is generally smaller and is by no means larger than thelargest distance within the lower measuring range the-anode current ofthe tube 2| increasesto thevalue which is sufficient for operating therelay l9, -despite the reduced sensitiveness; as a result of which theparallel connection ofthe resistance 2| with the relay l9-andsubsequently the short circuit ofthe resistance I4 is removed Thus themaximum frequency deviationis brought to the value corresponding to thelower measuring range. Howeven-the resultant-increase of the negativecon-- trol voltage suppliedto the grid of the tube 20 does not involverelease of the relay I9 since the sensitiveness of the relay l9 haspreviously been increased by cutting out the resistance 2 I. Thus it isavoided also in this case that the changeover is undone by the controlvoltage variation which appears in changing-over the measuring range.

We claim:

1. A radio-altimeter having first and second altitude measuring; ranges,the upper limit of the first range beingcommon to the lowerlimit of thesecond range, said altimeter comprising -means to transmitfrequency-modulated waves to be reflected from the earth, a rangeswitching circuit for abruptly changing the maximum frequency deviationof said waves from one value adapted ror the first range, to anothervalue adapted for the second range, areceiver for receivingthefrequency-modulated wave and for producingasignal corrnsspQn dingtothe differencegin frequency. of the transmitted and reflected Waves,means connected t ai re r for deriving a control voltagewhose amplitudeis proportional tov the frequency. of said signal, and arelay-systemoperated by saidcontrolvoltage and arranged to actuatesaid rangeswitching circuit, said relay systembbeingenergized by control voltageexcecdingthe magnitude thereof corresponding to..thefrequencyoi saidsignal obtained at the common limit of said ranges, said relay systemfurther including means responsive to the energization of said systemfor reducing the sensitiveness of said relay system to the appliedcontrol voltage.

2. A radio altimeter having low and high altitude measuring ranges, theupper limit of the low range being common to the lower limit of the highrange, said altimeter comprising means to transmit frequency-modulatedwaves to be reflected from the earth, range switching means for abruptlychanging the maximum frequency deviation of said waves from a one valueadapted for the low range to another value adapted for the high range, areceiver for receiving the frequency-modulated wave and for producing asignal corresponding in frequency to the difference in frequency of thetransmitted and refiected waves, means connected to said receiver forderiving a control voltage whose amplitude is proportional to thefrequency of said signal, and a relay system operated by said controlvoltage and arranged to actuate said range switching means, said systembeing responsive to a first predetermined magnitude of control voltageto efiect changeover from the low to the high range and to a secondpredetermined magnitude of control voltage to effect changeover from thehigh to the low range, the difierence between said first and secondpredetermined magnitudes of control voltage being greater than thevariation in the control voltage occurring on the changeover in therange.

3. A radio altimeter having low and high altitude measuring ranges, theupper limit of the low range being common to the lower limit of the highrange, said altimeter comprising means to transmit frequency-modulatedwaves to be reflected from the earth, said transmitter means includingan adjustable condenser for varying the frequency thereof,electromagnetic means to vibrate said condenser, a source of alternatingvoltage to energize said electromagnetic means at a prescribed rate, avoltage dropping resistor interposed between said source and saidelectromagnetic means and a switch in shunt relation with said resistor,whereby in the condition where said switch is closed the maximumfrequency deviation effected by said condenser is determined by theamplitude of said source and when said switch is open by the amplitudeof said source as reduced by said resistor, said switch being closed inthe low range and open in the high range, a receiver for receiving saidtransmitted waves directly and after reflection, means connected to saidreceiver for deriving from said receiver a, control voltage proportionalto the difference in frequency between the direc 1y received waves andthe reflected received waves, and a relay system arranged to actuatesaid switch, said system being responsive to a first predeterminedmagnitude of control voltage to effect changeover from the low to thehigh range and to a second predetermined magnitude of control voltage toeffect changeover from the high to the low range, the difference betweensaid first and second predetermined magnitudes of control voltage beinggreater than the variation in the control voltage occurring on thechangeover in the range.

4. A radio altimeter having low and high altitude measuring ranges, theupper limit of the low range being common to the lower limit of the highrange, said altimeter comprising means to transmit frequency-modulatedwaves to be reflected from the earth, said transmitted means 8 includingan adjustable condenser for varying the frequency thereof,electromagnetic means to vibrate said condenser, a source of alternatingvoltage to energize said electromagnetic means at a prescribed rate, avoltage dropping resistor interposed between said source and saidelectromagnetic means, and a first relay provided with a coil and acontact switch connected in shunt relation with said resistor whereby inthe condition where said contact switch is closed the maximum frequencydeviation effected by saidv condenser is determined by the amplitude ofsaid source and when said contact switch is open by the amplitude ofsaid source as reduced by said resistor, said switch being closed in thelow range and open in the high range, a receiver for receiving saidtransmitted wave directly and after reflection, means connected to saidreceiver for deriving from said receiver a control voltage proportionalto the difference in frequenc between the directly received waves andthe received reflected waves, a second relay having a coil and pair ofcontact switches and arranged for operation by said control voltage, a,shunt resistance connected in series with one of said pair of contactswitches across said coil of said second relay, and a potential supplyconnected in series with the other of said pair of contact switches andsaid coil of said first relay, said coil of said second relay in theabsence of said shunt resistance being responsive to the control voltagecorresponding to the signal obtained at the common limit of said ranges.l

5. An arrangement as set forth in claim 4 further including retardationmeans in conjunction with said first relay whereby the operation of saidfirst relay occurs after a time interval following the operation of saidsecond relay.

CORNELIS JOHANNES HENRICUS ANTONIUS STAAL. I-IENDRIK VAN DE WEG.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,222,586 Sanders Nov. 19, 19402,257,830 Wolff et al. Oct. '7, 1941 2,261,272 Newhouse Nov. 4, 1941

